1. Related Art
This invention relates to a magnetic thin film memory, and a recording and reproducing method; in which information is recorded using the direction of magnetization and the recorded information is reproduced utilizing Giant Magnetoresistance.
2. Prior Art
A semiconductor memory is a well-known conventional static solid memory. The semiconductor memory may be a volatile memory, such as a DRAM, which loses the recorded information if the power supply is cut off, or an involatile memory, such as a flash memory or ferroelectric memory, which do not lose the information even if the power supply is cut off.
In a DRAM, in the form of a ferroelectric memory, the recorded information is stored in a capacitor. The information is recorded by the storing or absence of a charge in the capacitor, or by the direction of the polarization. Thus, at least one transistor is needed per memory cell. In a flash memory, the information is recorded by changing the threshold voltage of a control gate dependent on whether a charge is stored or not in a floating gate. Thus, at least one transistor is also needed per memory cell.
The transistor is made by injecting an impurity element such as boron, phosphorus etc. in a Si crystal to form a p-type or n-type semiconductor. The Si crystal must have a structure with no defects in order to get appropriate values of the physical properties of the band structure. Also, a Si crystal is required having sufficient thickness to withstand injection of the impurity element. For these reasons, it is very difficult to form a Si film on the Si substrate, to produce the overlaid transistor.
Accordingly, in a conventional semiconductor memory, it has not been possible to overlie a plurality of memory cells in the direction of thickness of the memory.
A volatile magnetic solid memory utilizing Giant Magnetoresistance is now also being proposed. For example, U.S. Pat. No. 5,432,734 (corresponding to Japanese Patent Application No. 7-66033) shows the matrix structure of a magnetic memory as shown in FIGS. 1A and 1B, in which:
FIG. 1A is a plan view of the memory; and
FIG. 1B is a schematic sectional view along the line I--I in FIG. 1A.
A plurality of magnetoresistive (MR) devices 61 are provided as a matrix structure on the substrate 60. Each MR device 61 is composed of a Giant Magnetoresistive film comprising a non-magnetic layer between two magnetic layers. The digital data is stored by means of the direction of magnetization of the magnetic layer having the lower coercive force. The magnetization of the magnetic layer having the higher coercive force is oriented in a predetermined direction. The recording is performed by supplying an electric current into a word wire W1-W5 provided on the top of the magnetic layer and into a sense wire S1-S5, causing the portions at which the word wire and the sense wire cross each other to generate a big magnetic field. The reproducing is performed by reversing the direction of the magnetization of the magnetic layer having the lower coercive force and then detecting the resistance change. Thus, after reproducing, rewriting is required.
U.S. Pat. No. 5,432,734 describes a memory device connected in series arranged in one layer on the substrate. In this structure, it is impossible to increase the integration rate in the limited area available.
A highly integrated magnetic thin film memory in which the plurality of the magnetoresistive devices are overlaid is described in Japanese Patent Application Publication No. 10-116490. The structure of the magnetic thin film memory is shown in FIGS. 2A and 2B in which:
FIG. 2A shows a perspective view of the memory; and
FIG. 2B shows a sectional view along line II--II of FIG. 2A.
In FIGS. 2A and 2B, 70 indicates the magnetic thin film layers whilst 71 indicates the semiconductor substrate. 72a, b, c, d indicate Giant Magnetoresistive (GMR) portions. 73a, b, c, d, e, f indicate word wires. 74a, b, c, d, e indicate sense wires. 83 indicates a conductor linking the sense wires 74a, 74c. 84 indicates layers of a dielectric material. In this magnetic thin film memory, a plurality of GMR portions are overlaid in the direction of thickness of the memory. Therefore it is possible to write a number of bits in the size of one cell and to thus increase the rate of integration. However, in the magnetic thin film memory described in Japanese Patent Application Publication No. 10-116490, it has been difficult to increase the speed of readout out due to the influence of the wiring because the electric current flows in the direction parallel to the surface of the magnetic layer when reproducing the information.
Also, in the magnetic thin film memory described in Japanese Patent Application Publication No: 10-116490, if a third GMR portion is overlaid on the top layer of dielectric material 84, because the GMR portions in each layer must be connected in series as shown in FIG. 2A and FIG. 2B, another conductor must connect the sense wire of the GMR portions in such a third layer with the GMR portions 72c, d in the second layer arranged at the opposite side of the sense wires 74 to the conductor 83 so that the sense wire 74 of the second layer is connected with the sense wire of the GMR portions in the third additional layer. As a result, the conductor 83 must be staggered in each layer. This leads to an increase in production processing. and thus production costs.